A. Field of the Invention
The invention relates to scanners for providing a digitized image of the information in an object such as a letter or a photographic slide and, more particularly, to a color scanner and the calibration therefor.
B. Prior Art
Scanners are used to provide a digitized image of a reflective object such as a letter or a photograph, or a transmissive object such as a slide, etc. Hereinafter, these objects will be referred to collectivley as "documents". Color scanners present a particular challenge to the scanner designer, since it is essential to preserve the relative color values of the original throughout the many stages of processing required to form the desired digital image. In particular, the system electronics inherently includes analog components which are especially susceptible to drift; frequent recalibration is thus essential.
The calibration process itself is frequently complex. Calibration is typically initially performed at the factory, where sophisticated instruments facilitate the process. Even then, because of the unique demands of scanners, particularly color scanners, the process is often time consuming, and thus costly. Further, it is generally necessary to repeat the calibration process throughout the life of the scanner. Typically this is performed at least once at the start of a scanning run, and often is at least semi-automated. However, some systems lose calibration during the scan, and become increasingly unreliable as the scan proceeds. Others are still less than satisfactory when applied to the demanding environment of color scanner calibration.
A major source of error arises from the electro-optic detector system itself. Such systems generally comprise a detector head for receiving light reflected from or transmitted through a document, and for converting that light into electric signals indicative of the intensity of the reflected or transmitted light. The intensity of the reflected or transmitted light that is measured is a function not only of the "true" intensity arising from the characteristics of the image itself, but also from extraneous interference or noise caused primarily by undesired variations in the illumination and in the characteristics of the detector head.
Additional error arises in the electronic processing circuitry that receives the output of the electro-optic detector and converts to a usable output signal, generally in digital form. Such circuitry is subject to electronic noise and drift. The problem is particularly acute when logarithmic processing circuitry is desired because of the wide dynamic range of such circuitry and the inherent ambiguities presented by the circuitry at low light levels which also have high noise levels.